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Official: ISIS is al Qaeda alternative

Story highlights

The Islamic State, also known as ISIS, is expanding its presence, official said

Fighters loyal to ISIS could number more than 10,000 with some returning home countries

Law enforcement is watching a small number of Americans for possible affiliation

The U.S. is watching to see if ISIS loses support from other Sunnis

The Islamic State terror group is now "a credible alternative to al Qaeda" that is "expanding its presence" with foreign fighters returning from Syria, and possibly Iraq, to their home countries, a U.S. intelligence official said Thursday.

The official, who declined to be identified due to the sensitive nature of the information, has direct knowledge of the latest intelligence on the group, also known as ISIS or ISIL.

According to an assessment, the group has grown in size since the spring and its takeover of Mosul in northern Iraq as more fighters from around the world have mainly traveled to Syria to join its ranks.

The United States believes that while the group remains largely focused on its brutal takeover of large areas of Iraq, there is also an "expansion of its external terrorist ambitions."

Some foreign fighters are returning to their home countries with orders to "start new cells" of terrorist activity, the official said.

Iraqi refugees fleeing ISIS 17 photos

Iraqi refugees fleeing ISIS17 photos

Iraqi refugees fleeing ISIS – A Yazidi family from Sinjar cleans a spot for themselves in a derelict building that houses more than a thousand other refugees on Thursday, August 14, in Zakho, Iraq.

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Iraqi refugees fleeing ISIS17 photos

Iraqi refugees fleeing ISIS – A woman and child sit in the makeshift housing on Thursday.

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Iraqi refugees fleeing ISIS – A Yazidi woman holds her baby while crossing Peshkhabour bridge from Syria back into Kurdish-controlled Iraq on Tuesday, August 12.

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Iraqi refugees fleeing ISIS – Entire families carry nothing but the clothes on their back. Some are barefoot. And not everyone who set out on the arduous journey survived.

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Iraqi refugees fleeing ISIS – The militant group ISIS, which now calls itself the Islamic State, executes civilians who don't adhere to its version of Sunni Islam.

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Iraqi refugees fleeing ISIS – Yazidis fled into the barren and windswept Sinjar Mountains more than a week ago after ISIS captured their town.

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Iraqi refugees fleeing ISIS – Descendants of Kurds and followers of an ancient pre-Islamic religion, Yazidis are one of Iraq's smallest minorities, and have been persecuted for centuries, but they have a strong sense of community.

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Iraqi refugees fleeing ISIS – According to some accounts, Syrian Kurds also helped people use parts of northeastern Syria under their control to reach Kurdish areas of northern Iraq.

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Iraqi refugees fleeing ISIS – A young refugee carries a disabled man across the bridge.

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Iraqi refugees fleeing ISIS – Thousands trudge across a river to seek humanitarian aid in Syria.

Iraqi refugees fleeing ISIS – A senior Kurdish official estimated that as many as 70,000 people remain trapped on Mount Sinjar, and that at least 100 have died so far from dehydration and the heat. CNN could not independently confirm those estimates.

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Iraqi refugees fleeing ISIS – A man weeps after been reunited with his family.

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Inside the ISIS militant operation

While it currently has funds from taking over oil facilities and other operations in Iraq, the United States believes that will not be enough to sustain ISIS if it tries to seize the entire country, the official said.

The United States also is watching closely to see whether and at what point ISIS loses support from Sunni loyalists in Iraq.

The U.S. calculation is that ISIS is taking advantage of its recent momentum. Because it is seen as "successful" in many areas, including Yemen and Africa where al Qaeda affiliates operate, they have gained support from some jihadists more widely.